1. Technical Field
The present invention generally relates to methods and apparatus for carrying on communications over optical fibers. More specifically, the invention is directed to methods and apparatus to provide bi-directional telephonic communication and bi-directional digital data transmission such as digital subscriber line services and transmitting multicast TV.
2. Description of the Related Art
The communications industry is using more and more optical fibers in lieu of copper wire. Optical fibers have an extremely high bandwidth thereby allowing the transmission of significantly more information than can be carried by a copper wire transmission line such as twisted pairs or coaxial cable.
Of course, modern telephone systems require bi-directional communications where each station or user on a communication channel can both transmit and receive. This is true, of course, whether using electrical wiring or optical fibers as the transmission medium. Early telephone communication systems solved this need by simply providing separate copper wires for carrying the communications in each direction, and this approach is still used in older installations where telephony is the only required service. It is also often used even where digital transmission service is demanded as the signals get closer to the end users. Although twisted pairs and coaxial cables are used in homes and distribution terminals close to the home end user, some modern telecommunication systems now use microwave and optic fibers as transmission mediums.
Because of extremely high bandwidths available for use by an optical fiber, a single fiber is quite capable of carrying a great number of communications in both directions. One technique of optical transmission is WDM (wavelength divisional multiplexing) and uses different wavelengths for each direction of travel.
Another area of rapidly growing technology is providing unidirectional TV signals by cable to a multiplicity of subscribers or users (multicast). In the past, such signals were and still are typically transmitted by the use of coaxial cables (e.g. cable TV). However, the use of optical fibers for transmission allows broad band transmission to a large numbers of customers and, since substantially all of the transmission of TV signals is one way (i.e. unidirectional), if a single optical fiber were used solely for the TV signals there would be almost no use of the selected wavelength of light for carrying return signal, which are typically control or information signals.